1. Field of Invention
The present invention relates to a semiconductor device and a method of manufacturing the same, and specifically, to a semiconductor device having a capacitor, particularly, a dielectric film used for the capacitor and a method of manufacturing the same. Further, the present invention relates to a novel atomic layer deposition (ALD) method appropriate for introducing impurities in low concentration.
2. Description of Related Arts
As one of dielectric materials of a capacitor for a dynamic random access memory (DRAM), there is zirconium oxide (ZrO2).
The DRAM requires heat treatment at a temperature of about 450° C. to 500° C. as an inevitable process after formation of a capacitor. In this case, it is impossible to obtain sufficient thermal stability by using a monolithic dielectric film of a zirconium oxide film and there is a problem of increasing leakage current after heat treatment.
Accordingly, various attempts have been made to add thermal stability, and there are a ZAZ structure (TiN/ZrO2/Al2O3/ZrO2/TiN, Z and A of ZAZ represent a ZrO2 layer and an Al2O3 layer, respectively), a structure in which Al2O3 and ZrO2 films are stacked many times, and the like.
There structures intend to achieve the desired characteristics by combining zirconium oxide (ZrO2) having high permittivity and aluminum oxide (Al2O3) having excellent thermal stability despite low permittivity.
For example, JP 2006-135339A discloses a method of forming an AZ structure, a ZA structure, a ZAZ structure or a multi-layered dielectric film in which a ZrO2 thin film and an Al2O3 thin film are alternately stacked far DRAM wherein a feature size (F value: ½ of a minimum pattern pitch) is equal to or less than 70 nm.
In formation of the thin film, the ALD method is used, ZrCl4, Zr[N(CH3)C2H5]4, Zr(O-tBu)4, Zr[N(CH3)2]4, Zr[N(C2H5)(CH3)]4, Zr[N(C2H5)2]4, Zr(tmhd)4, Zr(OiC3H7)3(tmtd) and Zr(OtBu)4 are disclosed as a Zr source, and Al(CH3)3 and Al(C2H5)3 are disclosed as an Al source.
In the ALD method for obtaining a ZrO2 thin film, the steps of adsorbing a Zr source on a surface of a substrate, discharging a non-adsorbed portion of the Zr source from a reaction chamber by a purge gas such as N2 and Ar, oxidizing the adsorbed Zr source by a reaction gas such as O3, and purging an unreacted portion of the reaction gas by the same purge gas as above are repeated as many times as desired.
Similarly, for obtaining an Al2O3 thin film, the steps of adsorbing an Al source on a surface of a substrate, discharging a non-adsorbed portion of the Al source from a reaction chamber by a purge gas such as N2 and Ar, oxidizing the adsorbed Al source by a reaction gas such as O3, and purging an unreacted portion of the reaction gas by the same purge gas as above are repeated as many times as desired.
Further, JP 2007-73926A discloses “a dielectric film including a first dielectric film having a relative permittivity of at least 25, a second dielectric film formed on the first dielectric film using a material having a crystallization rate lower than that of the first dielectric film, and a third dielectric film formed on the second dielectric film using the same material as that of the first dielectric film.” It discloses a structure in which amorphous Al2O3 is present between crystallized ZrO2 films.
The formation of the ZrO2 film or Al2O3 film employs the same ALD method as in JP 2006-135339A. Zr(O-t-Bu)4, Zr[N(CH3)2]4, Zr[N(C2H5)(CH3)]4, Zr[N(C2H5)2]4, Zr(tmhd)4, Zr(OiC3H7)3(tmhd), Zr(OtBu)4 and Zr(OtBu)(C2H5CH3)3 are disclosed as a Zr source, and trimethylaluminum (TMA:Al(CH3)3), Al(C2H5) is disclosed as an Al source.
Further, JP 2007-281407A discloses, in order to obtain a tetragonal ZrO2 structure having high permittivity, adding an extra O3 step to an ALD sequence, a setting a temperature of a substrate to be 250° C. to 350° C., controlling O3 concentration of an oxidizing agent to be 150 g/m3 or more, or the like.
In this case, Zr(O-tBu)4, Zr[N(CH3)2]4, Zr[N(C2H5)(CH3)]4, Zr[N(C2H5)2]4, Zr(tmhd)4, Zr(OiC3H7)3(tmhd), and Zr(OtBu)4 are disclosed as a Zr source.
In addition, JP 2007-150242A discloses a method of manufacturing a capacitor having a ZrxAlyOz film in which zirconium, aluminum and oxygen are mixed at specific molar fractions of x, y and z by using an ALD method. In the ZrxAlyOz dielectric film, a sum of the molar fractions of x, y and z is 1 and the value dividing the molar fraction x by the molar fraction y ranges from 1 to 10 (0.091≦y/(x+y)≦0.50, i.e., a ratio of the number of atoms represented by Al/(Al+Zr) may range from about 9 to 50 atom %).
Further, the step of forming the ZrxAlyOz dielectric film includes the steps of:
introducing a Zr source and adsorbing the Zr source on the lower electrode,
removing a non-adsorbed portion of the Zr source by supplying a first purge gas,
introducing an Al source and adsorbing the Al source on the Zr source adsorbed on the lower electrode,
removing a non-adsorbed portion of the Al source by supplying a second purge gas,
forming the ZrxAlyOz dielectric film by reaction of the Zr source and the Al source adsorbed on the lower electrode by supplying a reaction gas, and
removing an unreacted portion of the reaction gas by supplying a third purge gas.
As the Zr source, it discloses ZrCl4, Zr[N(CH3)C2H5]4, Zr(O-tBu)4, Zr[N(CH3)2]4, Zr[N(C2H5)(CH3)]4, Zr[N(C2H5)2]4, Zr(tmhd)4, Zr(OiC3H7)3(tmtd), and Zr(OtBu)4.
Incidentally, JP 2007-150242A does not mention whether the obtained dielectric film is crystalline or amorphous. Further, it does not disclose how to control molar fractions within the specific range.
The DRAM stores 1 bit in a unit cell configured as one transistor and one capacitor. As the number of bits increases, an occupation area per unit cell tends to be reduced. Currently, the generation of DRAM is shifted to F value of 40 nm or less, the occupation area per unit cell becomes more and more small.
Since the storage capacitance of a capacitor requires a predetermined amount (20 fF to 25 fF), although the occupation area per unit cell becomes smaller, it is required to ensure a predetermined amount of the storage capacitance. Accordingly, a steric structure of a capacitor has been developed in order to expand an electrode area, and an aspect ratio of the structure has been increased to exceed 30 in order to raise an electrode in a vertical direction of the substrate.
However, in a capacitor for DRAM since F value of 40 nm, it is considered that an aspect ratio of 35 is a limitation achievable by one dry etching in the current processing technology.
Therefore, in order to obtain a necessary storage capacitance of the capacitor, it is required to maintain leakage current of the capacitor to be equal to that of a conventional case (1E-7 A/cm2 or less) and to make an equivalent oxide thickness (EOT) (value calculated by converting capacitance of the capacitor per unit area into an equivalent silicon oxide film thickness) smaller than a conventional case, i.e., to be equal to or less than 0.9 nm.
As described above, in order to realize a small EOT and small leakage current in a dielectric film formed for an electrode having a steric structure, it requires a capacitance film (dielectric film) having high permittivity, good coverage and sufficient thermal stability. Actually, these have a trade-off relationship.
1) Trade-Off Between Permittivity and Coverage
For example, since the permittivity of the amorphous ZrO2 film is low, it is necessary to obtain the crystallized ZrO2 film in order to obtain the capacitance film having high permittivity. Particularly, in order to obtain the ZrO2 film having high permittivity and a tetragonal structure, as disclosed in JP 2007-281407A, it is necessary to form a film at a relatively high temperature.
However, the Zr source disclosed in the above-mentioned prior art documents is self-decomposed by heat in the film formation at a high temperature at which a tetragonal structure is obtained, and coverage is deteriorated. As a result, it has been found by the present inventors that it cannot be applied to the steric structure having an aspect ratio of 20 or more.
If the other conditions are the same, since the leakage current depends on the thickness at the thinnest portion of the dielectric film, the deterioration of the coverage causes non-uniformity of film thickness, and the film thickness of the dielectric film should be raised by a corresponding amount. Consequently, since it is impossible to reduce the EOT, the permittivity is hardly to be compatible with the coverage.
2) Trade-Off Between Thermal Stability and Permittivity
Further, in order to realize necessary thermal stability, it is required to set an Al amount introduced as an impurity and control its amount. This is because if the amount of Al is excessively large, it is hardly to obtain a film having high permittivity, and if the amount of Al is excessively small, it is hardly to obtain sufficient thermal stability.
The present inventors have conducted the same experiment again, and it could be found that sufficient thermal stability can be obtained, but crystallization of the ZrO2 film is difficult in a range of Al concentration disclosed in JP 2007-150242A, and, thus, it is hardly to obtain a small EOT that may correspond to a device since F value of 40 nm.
Further, with regard to the amount of Al, not only an average concentration in the entire dielectric film, but also a local density is important. Unlike a PVD method or CVD method in which impurities can be relatively uniformly dispersed to a base material, it is general in addition of impurities by an ALD method that the concentration of impurities is formed in a film thickness direction by a film formation method unless the impurities are dispersed to the base marital due to a high temperature. However, in the film formation at a high temperature, the coverage is deteriorated as described in 1) above.
Meanwhile, there is a phenomenon generally called “size effects” in the crystallized dielectric film. As the film thickness decreases, the permittivity tends to decrease. In case of zirconium oxide, this phenomenon becomes severe in a physical film thickness smaller than about 6 nm.
For example, in a case where the ZrO2 film is formed by the ALD method, and the Al2O3 film is formed by the ALD method in the same way during the film formation, if the area density of Al2O3 is higher than a certain value, ZrO2 cannot be crystallized over the Al2O3 layer. Accordingly, the ZrO2 crystal grains are separated vertically by the Al2O3 layer, and the ZrO2 film is divided into vertically separate layers by the Al2O3 layer. As a result, even though a total film thickness is 6 nm or more, the permittivity of each of the ZrO2 films divided by the Al2O3 layer is reduced by the size effects, and it is difficult to make the EOT of the total dielectric film small.
The present inventors have verified that in a combination of TMA serving as an Al source and a Zr source disclosed in the conventional technology, it is impossible to prevent division of the ZrO2 film even by Al doping in which the Al2O3 layer is formed by one ALD cycle as disclosed in JP 2007-73926A.
Further, although Al doping was performed by selecting one ALD cycle of a ZrxAlyOz film disclosed in JP 2007-150242A and using the Zr source disclosed in the conventional technology, it was impossible to suppress the division of the ZrO2 film.
As described above, it is difficult to avoid the division of the ZrO2 film in the ZAZ structure by a conventional combination of the Zr source and the Al source and a conventional sequence.
Therefore, in order to obtain a small EOT, it is necessary to clarify a value of “area density of Al per one ALD cycle” to prevent the ZrO2 film from being divided by an Al-doped layer, and find a means to realize the value.